linux-kernel - Re: [PATCH V6 6/8] x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS

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Message-ID: <YnAbQK9kLdCxxGSf@zn.tnic>
Date:   Mon, 2 May 2022 19:56:16 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH V6 6/8] x86/entry: Convert SWAPGS to swapgs and remove
 the definition of SWAPGS

On Mon, May 02, 2022 at 02:18:35PM +0200, Juergen Gross wrote:
> The syscall will land in the hypervisor, which then will activate
> the related registered callback (xen_sysenter_target).

Aha, that would do a bit of a fixup and then go to the common label
entry_SYSENTER_compat_after_hwframe.

We probably should document this so that it is clear what uses those...

---
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9da5eef9affe..93065c4914ad 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -103,6 +103,11 @@ SYM_INNER_LABEL(entry_SYSCALL_64_safe_stack, SYM_L_GLOBAL)
 	pushq	%r11					/* pt_regs->flags */
 	pushq	$__USER_CS				/* pt_regs->cs */
 	pushq	%rcx					/* pt_regs->ip */
+
+	/*
+	 * XENPV jumps here after frame fixup in the respective entry points in
+	 * ...xen-asm.S.
+	 */
 SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
 	pushq	%rax					/* pt_regs->orig_ax */
 
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index c5aeb0819707..b473f8e17afc 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -71,6 +71,11 @@ SYM_CODE_START(entry_SYSENTER_compat)
 	pushfq				/* pt_regs->flags (except IF = 0) */
 	pushq	$__USER32_CS		/* pt_regs->cs */
 	pushq	$0			/* pt_regs->ip = 0 (placeholder) */
+
+	/*
+	 * XENPV jumps here after frame fixup in the respective entry points in
+	 * ...xen-asm.S.
+	 */
 SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
 
 	/*

> I wouldn't insist on the CONFIG_DEBUG_ENTRY chunk.

Ok, simpler asm, cool, let's do it then.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette